1. Field of the Invention
The present invention generally relates to a security sensor system, including a beam projecting unit for projecting a sensing beam such as an infrared beam and a beam receiving unit for receiving the sensing beam, in which the presence or absence of an object such as a human body when the object intercepts the sensing beam is detected.
2. Description of the Prior Art
As is well known to those skilled in the art, this type of security sensor system generally includes a beam projecting unit having a beam projector for projecting a sensing beam such as an infrared beam and a beam receiving unit having a beam receiver optically aligned with the beam projector for receiving the sensing beam. An area between the beam projector and the beam receiver is defined as an alert area of a rectilinear distance, which may extend from a few meters to a few hundred meters. When an unauthorized person enters the alert area and intercepts the sensing beam, the detection is made.
In order for the security sensor system to work satisfactorily, it has hitherto been well known in the art to provide the beam projecting unit with an optical power adjustor for adjusting the beam projecting power of the beam projector to keep the sensing beam at a constant level of power in dependence on the level of light (sensing beam) received by the beam receiver, so that the beam receiver may receive a proper amount of the sensing beam at all times. According to this known security sensor system, such adjustment of the beam projecting power of the beam projector is carried out during the installation or maintenance of the system to align the respective optical axes of the beam projector and receiver. See, for example, the Japanese Laid-open Patent Publication No. 2002-367045.
On the other hand, it has often been observed that the level of light received by the beam receiver varies considerably with passage of time under the influence of external environment. By way of example, when it rains or snows, or when mist hangs in the air, the level of light received by the beam receiver attenuates as shown in FIG. 4. In order for the security sensor system to be substantially immune from such an influence, it is a general practice to set the beam projecting power of the beam projector to a rather higher value than a predetermined or required level.
It has, however, been found that if the beam projecting power is set to the higher value than the required level, the security sensor system may not work satisfactorily. More specifically, when the rain or snow clears up, a rain-soaked ground surface or a mantle of snow reflects the sensing beam, causing the beam receiver to receive the reflected sensing beam out of the path of the sensing beam traveling from the beam projector towards the beam receiver. Once this occurs, the security sensor system may fail to detect an unauthorized entry of a person within the alert area and to emit the warning signal, even though the person entering the alert area intercepts the sensing beam.
Also, when the security sensor system is installed in the vicinity of the wall, an effect similar to that described above may occur as the wall reflects the sensing beam out of the path.
Accordingly, the prior art security sensor system must be installed distant from the ground surface or the mantle of snow or the building wall, and the place of installation of the device is thus limited. Also, unless the beam receiver is set to a proper beam receiving level, a false warning signal may be likely to occur.
On the other hand, in order for an electric analog signal of a high level to be converted into a digital signal with an analog-to-digital (A/D) converter, used in the beam receiving unit to detect the beam receiving level, without being accompanied by an over-range, the A/D converter must have an input voltage range that is sufficiently broad. However, setting the broad input voltage range reduces the resolving power, worsening the detecting accuracy of the light receiving level. Also, while the automatic gain control (AGC) is often applied to an amplifier circuit employed in the beam receiving unit to secure an input dynamic range of the A/D converter, the application of the automatic gain control tends to make the beam receiving unit to be unable to achieve an accurate detection of the beam level such as weak input, strong input or reflected beam.